As is already known, such a speed reducer consists of an electric motor, having a rotor shaft which is extended beyond the motor itself and which carries a worm located in a hollow casing closed by a cover. Within the hollow interior of the casing is a speed reducing mechanism comprising a toothed wheel which is driven in rotation by the worm, and which drives, through an eccentric crank pin carried by the toothed wheel for free rotation and axial movement, a crank and connecting rod system which is coupled in a fixed manner to the said crank pin, the speed reducing system being such as to convert the continuous circular motion of the toothed wheel into alternating or oscillating rotary motion of an output shaft, which is carried by the casing and passes through the base of the latter so as to drive a wiper linkage.
Such a speed reducer must be able to transmit very large forces, and it is necessary to immobilise the crank pin carried by the toothed wheel against axial translational movement, by mounting it in such a way that the crank pin is unable to slide axially under the force transmitted by the toothed wheel. Such axial movement would damage the cover and set up unacceptable noise, besides causing the system and the output shaft to move in corresponding axial movement because of the rigid coupling with the crank and connecting rod system. This in turn gives rise to risks of damage and rubbing between the crank and connecting rod system and the cover, producing further noises, either in the form of audible bumps or grating noises.
It has already been proposed, for example in the specification of published German patent application No. DE 3 247 421A, to use the cover itself as an abutment surface for the upper end of the eccentric crank pin carried by the toothed wheel. For this purpose, the upper end of the crank pin is provided with an engagement means in the form of a cap of plastics material which engages on the inner surface of the cover, and on the surface of the connecting rod of the crank and connecting rod system facing the cover. The plastics material of this cap is one having a low coefficient of friction, so that the latter can engage with low friction on the cover without causing any significant frictional heating, and without adversely affecting either the rotation of the crank pin in the housing which is provided for this purpose in the toothed wheel, or its circular movement caused by the toothed wheel.
However, this arrangement, though effective, is relatively expensive in terms of assembly costs, and even so, the friction which it generates is not negligible. In the assembly operation, the cap is a very small component, which is not easy to fit on to the end of the eccentric crank pin, and the cap is liable quite easily to become separated from the latter during subsequent manipulations in the assembly process.
In addition, the cap is trapped between the inner face of the cover and the surface of the connecting rod that faces towards the cover. Having regard to manufacturing tolerances, this can produce quite a large axial thrust against the cap, giving rise to frictional braking forces at the surfaces in contact with the cap. This is detrimental to efficient operation of the assembly. Again, it is necessary to provide a large number of different sizes of caps for different types of speed reducer.